The invention relates to a device for coating a web of material that travels around a backing roller.
Jet-coating devices are preferable when low coating densities (&lt;7 g/m.sup.2) are to be attained at high paper speeds (&gt;700 m/min). Essential components of known jet coaters are
a coating chamber that opens toward the backing roller and that is supplied with liquid coating from a preliminary-distribution chamber through a gap that extends across the operating width and PA1 a metering system in the form of a doctor that closes off the coating chamber at the paper-exit end.
The coating chamber is demarcated at the intake end by an overflow plate that terminates near the backing roller, leaving an overflow gap. The liquid flowing off through the overflow gap prevents air from entering the coating chamber along with the web. The chamber is sealed off at the sides by backup plates for example at the ends of the backing roller.
The structures, including the blade and its support (the blade beam), that demarcate the coating chamber constitute in the known coaters one structural unit that can be pivoted around an axis paralleling that of the backing roller. This integrated unit can be pivoted to vary the coating angle of the blade and accordingly the density of the coating.
A generic coater of this type is described in German GM No. 8 414 904. The coating density can additionally be varied in this coater by varying the distance of the doctor beam from the backing roller.
Another generic device is disclosed in German Pat. No. 3 336 553.
A serious drawback to the known jet coaters is that the coating density can be varied only within narrow limits because it is possible to affect it only to a limited extent by varying the coating angle of the blade. Since, that is, varying the angle has a negative effect on the flow conditions in the coating chamber, coating defects will occur as the result of undesirable turbulence, coating separation, and the invasion of air.
These negative consequences can, to the extent that they derive from changes in the overflow gap, be eliminated only by the complicated process of adjusting the dimension of the gap (at the overflow plate for example). Also necessary is, depending on the particular embodiment, a complicated readjustment of the lateral backup plates and of the structures that demarcate the edge of the coating.
Paper coaters in which the coating chamber is completely separated from the metering system are known. In this embodiment it is necessary to ensure an overflow of liquid coating from the coating chamber at both the paper-intake and paper-exit ends. Since this, however, leads to too high excess-coating rates, the paper speed is limited by the requisite head.